Cooling gases containing condensible material



June 24, 1952 P. c. KEITH 2,601,298

COOLING GASES CONTAINING CONDENSIBLE MATERIAL Filed Feb. 8, 1949INVENTOR.

Percival 6 [1217/0 I] TTOIP/VE Y Patented June 24, 1952 COOLING GASESCONTAINING CONDENSIBLE MATERIAL Percival C. Keith, Peapack, N, J.,assignor to Hydrocarbon Research, Inc. New York, N. Y., a corporation ofNew Jersey Application February 8, 1949, Serial No. 75,203

14 Claims. 1

This invention relates to processes involving; heat exchange and moreparticularly to processes for cooling gaseous streams in whichcondensation occurs during the contact of'the gaseous stream with thecooling surfaces.

In heat exchange procedures, condensible components of a gaseous streamare deposited on the surfaces of the cooling means when the gaseousstream is cooled below the condensation point of the condensiblecomponents. Often the cooling surface is cold enough to cause freezingor solidification of the condensed material so that there is aprogressive building-up of condensed material on the cooling surfaceswith the result that the heat transfer rate decreases and the heatexchange apparatus is stopped up or choked with the solidified deposits.Several proposals have been made to prevent or remove such solidifieddeposits.

It has been suggested that the gaseous stream to be cooled be pretreatedto remove .condensible components prior to admittance of the gaseousstream into the cooling exchanger. Flor example, in the production ofoxygen by the liquefaction and rectification of air where air feed iscooled by passage in indirect heat exchange relation with the .co'ldoutgoing products of rectification. the air which invariably containsabout 0.03% by volume of carbon dioxide and varying quantities ofmoisture, .is pretreated in driers and caustic scrubbers to remove thewater and carbon dioxide, respectively. prior'to admittance of the airinto the heat exchan ers. Even with such pretreatment which is costlyand cumbersome, the exchangers have to be thawed out periodically toremove solidified deposits of water and carbon dioxide which are presentin traces in the pretreated air.

More recently there has been developed the use of reversing exchangersby which t l-. aseous stream .to be cooled and the cooli flu d pa ed inindirect heat exchan e relation t ewith are reversed, i. e., the flowpaths are switched periodically so that the cooling fluid flows throughthe path previousla. traversed by the gaseous stream to be cooled andthe gaseous stream to be cooled flows through the path previously trailersed by the cooling fluid. By such reversal, th d po it d c mpensat arere vaporated 0r sublimed and carried out of thcheat exchanger by thecooling fluid. in other words, the ma terial deposited by the processstream durin one period of operation is reevaporated .and purged fromthe exchanger by the reverse flow-of the coolantstr-eain during thesucceeding period.

lea-114.2

The. use of reversing heat exchangers in a process in which the processgas is compressed to a relatively high pressure results in more costlyoperation from the standpoint of horsepower requirements because, uponevery reversal, the volume of the compressed process gas in the heatexchanger is lost and must be replaced.

It is a primary object of this invention to provide a simple andimproved process for cooling a gaseous stream to a temperature at whichcondensation normally occurson the cooling surfaces.

Another important object of this invention is to prevent the depositionon cooling surfaces of condensible components of a gaseous streamundergoing cooling.

It is a further object of this invention to provide a trulycontinuousmethod of cooling a gaseous stream containing condcnsible materialwithout pretreatment of the stream.

Other objects and advantages of this invention will become apparent fromthe description which follows. 3

In accordance with this invention, .adsorptive particles are broughtinto contact with the cooling surfaces of a heat exchanger to adsorbpreferentially the condensible matter .of the gaseous stream undergoingcooling in order to prevent the accumulation of condensed material onthe cooling surfaces. Adsorption particles laden with the .condensiblematter are periodically or continuously withdrawn from the heatexchanger and fresh adsorptive particles introduced to replace thosewithdrawn. The withdrawn adsorptive particles may be regenerated andrecycled to the heat exchanger. Since a gaseous stream containing one ormore condensible components can be cooled by this inventionsubstantially without condensation on the cooling surfaces, it isevident that the rate of heat transfer will not diminish in the courseof operation which, incidentally, does not need to be interruptedbecause .of stoppage of the exchanger or reversal of the streams passingtherethrough.

The present invention is particularly applicable in the cooling of agaseous stream containing at least one condensible component to atemperature at which the condensed material freezes or solidifies on thecooling surfaces.

The process of the invention finds valuable utility especially in theproduction of oxygen by the liquefaction and rectification of air. Forconvenience, further description of the invention will be in terms ofordinary air which is cooled foran oxygen plant, but it will beunderstood that the 'vessel H] through outlet 23.

invention is not limited to this illustrative application. Other typicalapplications include the cooling of hot gases containing tarry vapors inwhich case adsorptive carbon may be used to prevent the deposition oftar on the cooling surfaces, and the cooling of natural gas containingcondensible impurities like hydrogen sulfide and carbon dioxide in whichcase adsorptive bauxite and silica gel may be used, respectively, toobviate the accumulation of these components on the cooling surfaces.

In some cases, it may be advisable to use two or more differentadsorbents to remove different condensible components from the gaseousstream undergoing cooling. The different adsorbents may be used as aparticulate mixture or separately in different zones of the heatexchanger particularly where the different components are condensed indifferent temperature ranges. Sometimes, one particulate adsorbent maybe used to take up two or more condensible components of a gaseousstream which is being cooled to a temperature below the condensationpoints of these components.

For a clearer and more detailed understand- 'ing of this invention,reference is now made to the drawing forming a part of thisspecification, of which:

Figures 1 and 2 are schematic elevations in section of different formsof heat exchangers suited for the process of the invention. It is to beunderstood the invention is not limited to these illustrativeembodiments nor to others herein discussed.

The apparatus illustrated in Figure 1 comprises an upright cylindricalor oblong vessel H) with head I l to which pipe 12 is attached for theadmission of a warm air stream containing carbon dioxide and moisture,these components of air being normally condensible within the operatingtemperature range of the apparatus. Pipe [3 for the admission ofadsorptive particles to adsorb preferentially the condensible componentsis similarly attached to head H. Within vessel Hi there is a verticalbundle of cooling conduits M to cool the air stream. Coolant flowsupwardly through conduits l4, entering at the bottom thereof throughinlet l5 and manifold I6 and emptying through manifold H and outlet H3at the upper end thereof. The air stream and adsorptive particles are indirect contact with one another and with the exterior surfaces ofcooling conduits l4 and pass downwardly through vessel H3 incountercurrent relation to the upwardly flowing coolant within conduitsl4. During the downward passage, the adsorptive particles adsorb themoisture and carbon dioxide from the air and prevent the accumulation ofthese condensible components on the surfaces of cooling conduits 14,thereby avoiding a decrease in the heat transfer rate and stoppingup ofthe apparatus. The cooled air stream, now substantially free of moistureand carbon dioxide, becomes separated from the adsorbent particles byentering the inlets IQ of manifold and exits through outlet 2| whence itmay flow to a conventional rectification system to produce oxygen. Thecold adsorbent particles containing adsorbed carbon dioxide and waterare withdrawn from the lower tapered portion 22 of The withdrawnadsorbent particles may be regenerated and then returned to vessel In byway of inlet l2.

Referring to the form of apparatus typified by Figure 2, vessel H0 maybe of such shape that 4. its horizontal cross-section is rectangular orcircular. Vessel I I0 has a tapered bottom HI to which conduit H2 isconnected for the introduction of a warm air stream containingcomponents condensible at the normal operating conditions of theapparatus. Within vessel H0 there is a plurality of verticalcoolant-carrying conduits H4 to cool the incoming air stream. ConduitsH4 are fed with cooling fluid through inlet H5 to manifold H8 connectedto the upper ends of these conduits. The cooling fluid travelscountercurrent to the air stream, i. e., in a downwardly directionthrough conduits H4, and leaves at the bottom thereof through manifoldHT and outlet H8. The adsorptive particles, to prevent accumulation ofcondensible air constituents on the surfaces of conduits H4, aremaintained in contact with these surfaces in a plurality of horizontalzones H3 within vessel H0. These zones are separated from one another byperforated plates, grids or like porous horizontal separators H311permitting the upwardly flowing air stream and suspended adsorptiveparticles to pass therethrough but preventing the downward passage ofthe adsorptive particles. The adsorptive particles are of such sizethat, preferably, a state of dense phase fiuidization is readilymaintained with an upfiowing air stream having a velocity in the rangeof about 0.2 to 3.0 feet per second, preferably about 0.5 to 1.5 feetper second. Usually, the particle sim of the adsorptive particles is notgreater than about 20 mesh, and more frequently, not greater than 60mesh.

The air stream travels upwardly through a plurality of fluidized beds H3in indirect heat exchange relation with the countercurrently flowingcoolant in conduits H4 and the fluidized adsorbent particles adsorb thenormally condensible components therefrom. The cooled air streamrelatively free of moisture and carbon dioxide emerges from thepseudo-liquid level I l3b of the uppermost fluidized bed H3 and passesthrough separating means H9 to keep back any adsorptive particlesentrained therewith. The cold air, thus separated from adsorptiveparticles, leaves vessel H0 through outlet I20 attached to dome I2I. Theplurality of fluidized beds H3 permit the maintenance of a temperaturegradient along the vertical dimension of vessel Hi3, which gradient isnecessary for satisfactory operation of the exchanger. Each bed H3 has asubstantially uniform temperature but each succeeding bed has a lowertemperature than the adjoining lower bed. Thus, the lowest bedtemperature is at the top of vessel H0 adjacent the upper ends ofconduits H4 where the cooling fluid enters and the highest bedtemperature at the bottom of vessel H0 where the warm air stream entersby way of inlet I I2. There is, therefore, a descending temperaturegradient in the air stream in the direction of its flow. The fluidizedadsorptive particles preferentially adsorb condensible materials fromthe incoming air stream at different temperature levels of theexchanger, depending on the condensation temperature of these materials,and thereby prevent the accumulation of condensed materials on thesurfaces of conduits H4.

Adsorptive particles containing adsorbed material are periodically orcontinuously withdrawn from vessel H0, regenerated in any of theconventional ways to eliminate the adsorbed material and returned tovessel HD to adsorb again condensible components of the air streamundergoing cooling. For instance, as shown diagrammatically, adsorptiveparticles carrying carbon dioxide are withdrawn from the coldest(uppermost) fluidized bed H3 through line I22, heated in chamber 123 todrive off the adsorbed carbon dioxide which is eliminated through ventI24, and the purged adsorptive particles returned by way of line I to alower fluidized bed II3 which is at a temperature above that at whichcarbon dioxide would normally begin to condense, i. e., generally atemperature above about -180 F. It is advisable to bring the regeneratedadsorptive particles to the approximate temperature of the fluidized bedI I3 into which they are to be introduced before their entry into thebed. Similarly, adsorptive particles containing adsorbed moisture arewithdrawn through line I26 from a fluidized bed H3 at a temperaturebelow that at which substantially no water vapor is found in theupflowing air stream, say below about 0 Ft,

heated in chamber I21 to eliminate the adsorbed moisture which isdiscarded through line I28, and the regenerated particles returned byline I29 to the lowermost fluidized bed II3. If desired, chambers I23and I21 may be replaced by a single chamber in which the adsorptiveparticles may be purged of both adsorbed moisture and carbon dioxide.

The present invention may be carried out using various other types ofheat exchanging apparatus and procedures. For example, the adsorptiveparticles may be rapidly swept through an exchanger by suspension in theair stream to be cooled. Deposition of carbon dioxide and moisture inthe air on the cooling surfaces of the exchanger would be substantiallyavoided by the entrained adsorptive particles which take up thesecondensible components of air. The adsorptive particles carrying thecondensible components would be separated from the chilled air uponleaving the exchanger, would be regenerated and then would beresuspended in the warm air stream entering the exchanger to adsorbadditional quantities of condensible components.

For further clarification of the invention and its advantages, aspecific example will now be presented illustrating one embodiment ofthe the rectification is preferably conducted in two stages at differentpressures. The refrigeration necessary for liquefaction is supplied tothe air, after it has been compressed and water-cooled to approximatelyroom temperature, by indirect heat exchange with the cold eilluentproducts of rectification. An additional amount of refrigeration isusually supplied to compensate for cold losses resulting from thedifference in enthalpy between the incoming air and the outgoingproducts of rectification and for heat leaks in the system.

Cold nitrogen from the low-pressure stage of the rectification systementers heat exchanger conduits IM through inlet H5 and manifold H6. Atinlet N5, the nitrogen stream usually has a temperature in the range ofabout 270 F. to about 290 F. and is at a gauge pressure of about 2 to 6pounds per square inch. The incoming air stream containing moisture andcarbon dioxide is introduced into the lower portionof the exchangerthrough conduit H2 and fiows countercurrently in indirect heat exchangerelation to the nitrogen while fluidizing adsorptive particles .in aplurality ofbecls II3.

ill

At inlet I I2, the air stream generally is at a temperature in the rangeof about 30 to F. and at a gauge pressure of about 60 to 150 pounds persquare inch. As the air stream flows up through vessel H0, moisture isremoved from the stream by the fluidized adsorptive particles in thelower portion of vessel Ill! and carbon dioxide is removed bytheadsorptive particles in the upper portion of vessel III]. Thus, in itsupward flow, the air stream is cooled and freed of its condensiblecomponents which are taken up by the fluidized adsorptive particles;consequently, the external surfaces of the cooling conduits H4 aremaintained substantially free of deposits of these condensiblecomponents and a relatively high rate of heat transfer through the wallsof conduits I M is realized. The cooled air stream leaves vessel III] byway of outlet I26 after passing through filter H9 for the removal ofentrained adsorptive particles. The cooled air stream exiting fromoutlet I20 is generally at a temperature of about 5 to 15 F. above thetemperature of the nitrogen entering inlet H5 and thence flows to. thehigh-pressure stage of the rectification system wherein the air isseparated into oxygen and nitrogen product streams. Similarly, thenitrogen which is warmed in its passage through conduits II4 dischargesfrom outlet IISS at a temperature close to that of the air entering atinlet II2; usually the nitrogen exit temperature is about 5 to 15 F.below the air inlet temperature.

In one particular design, vessel H0 is about 33 feet high and haseighty-seven beds I I3 of fluidized silica gel particles. The airentering at inlet IIZ is at a temperature of 32 F. and a gauge pressureof 100 pounds per square inch. Air substantially free of moisture andcarbon dioxide leaves vessel IIB by way of outlet I20 at a temperatureof 260 F. and a gauge pressure of pounds per square inch. Vessel III]has two sets of cooling conduits H4, one set for the oxygenrectification product and the other for the nitrogen rectificationproduct. Both rectification product streams enter their respectiveinlets H5 at a temperature of -275 F. and a gauge pressure of about 5pounds per square inch and exit from their respective outlets H8 at atemperature of 26 F. and at nearly atmospheric pressure.

Silica gel containing adsorbed moisture is withdrawn by line I25 from afluidized bed H3 which is at a temperature of about 1-0 F., is heated inchamber I27 to expel the adsorbed moisture and then returned by line I29to the lowermost bed H3. The rate of withdrawals of silica gel toeliminate moisture from vessel I I0 is of the order of 1 pound of silicagel for each 10 pounds of air entering at inlet I I2.

Silica gel containing adsorbed carbon dioxide and traces of acetylenecarried into vessel IIB by the air stream is withdrawn from theuppermost fluidized bed I I3 by line I22, is heated in chamber I23 todrive off the adsorbed impurities and returned by line I25 to thefluidized bed ll|3 which is at a temperature of about F. The rate ofwithdrawal of silica gel to eliminate carbon dioxide and acetylene fromvessel III! is of the order of 0.7 pound of silica gel for each 10pounds of air entering at inlet I I2.

From the foregoing example, it is clear that the invention provides asimple and efiective process by which simultaneously a gaseous streamcontaining condensible components is continuously cooled by contactingcooling surfaces to a temperature below the freezing points of thecondensible components and the cooling surfaces are kept substantiallyfree of solidified deposits of the condensible components. Theadsorptive particles which, pursuant to the invention, contact both thegaseous stream and the cooling surfaces not only make'the operation ofthe heat exchanger very satisfactory since a higher rate of heattransfer is realized and interruptions from choking of the exchangerwith solidified deposits of condensible components are greatly minimizedbut also permit the withdrawal of the chilled gaseous stream in a highlypurified state relative to the contaminating condensible components.

The particle size of the solid absorbent selected for a given operationwill depend on whether the particles are to be fluidized or carried bygaseous entrainment or moved as a permeable bed. Usually, particle sizesbelow about 60 mesh are advisable where the particles are to befluidized or carried in suspension by a gas and coarser particles, sayabout 1 5 to inch in diameter, are better suited for forming a permeablebed. In a fluidized operation, it is generally desirable to maintain thefluidizing gas at a linear velocity of about 0.5 to 1.5 feet per second,while linear velocities of about 20 feet per second and higher arefrequently employed where the gas sweeps or entrains the adsorptivepowder through the heat exchanger.

Various modifications of the invention will occur to those skilled inthe art upon consideration of the foregoing disclosure without departingfrom the spirit or scope thereof. Accordingly, only such limitationsshould be imposed as are indicated by the appended claims.

What is claimed is:

1. In the method ofcooling a gaseous stream comprising condensiblematerial by passage of said stream over cooling surfaces at atemperature which is effective for the deposition of said condensiblematerial in frozen form on said cooling surfaces, the improvement ofsubstantially preventing said deposition of frozen condensible materialwhich comprises moving over said cooling surfaces a particulateadsorbent adapted to adsorb preferentially said condensible material,While passing said stream over said cooling surfaces.

2. The method of claim 1 wherein the moving particulate adsorbent iscarried in suspension by 4. The method of claim 3 wherein theparticulate adsorbent maintained in a fluidized state is held as aplurality of fluidized, vertically contiguous beds and said coolingsurfaces extend through said plurality of fluidized beds.

5. The method of claim 3 wherein the gaseous stream comprisingcondensible material is an air stream comprising carbon dioxide.

6. The method of chilling air containing moisture and carbon dioxide toa temperature below the freezing points of moisture and carbon dioxideand of separating said moisture and said carbon dioxide from said air,which comprises passing over cooling surfaces said air and adsorptiveparticles adapted to adsorb preferentially moisture and carbon dioxideto effect simultaneously the chilling of said air to a temperature belowthe freezing points of moisture and carbon dioxide and the adsorption ofsaid moisture and said carbon dioxide by said adsorptive particles, andseparating the chilled air substantially free 8 of said moisture andsaid carbon. dioxide from said adsorptive particles.

'7. The method of claim 6 wherein the adsorptive particles are silicagel.

8. The method of recovering the cold content of a product ofrectification in the liquefaction and rectification of air to produceoxygen, which comprises passing upwardly a stream of air containingmoisture and carbon dioxide through a fluidized mass of adsorptiveparticles adapted to adsorb preferentially moisture and carbon dioxide,said fiuidized mass being in contact with one side of heat transfersurfaces, passing downwardly a stream of rectifioationproduct at aninitial temperature below about 270 F. in contact with the other side ofsaid heat transfer surfaces, thereby chilling said air to a temperaturebelow the freezing point of carbon dioxide while substantiallypreventing the deposition of said carbon dioxide and said moisture onsaid heat transfer surfaces, withdrawing adsorptive particles containingadsorbed moistureand carbon dioxide from said fluidized mass, treatingthe withdrawn adsorptive particles to effect elimination of saidadsorbed moisture and carbon dioxide, and returning the treatedadsorptive particles to said fluidized mass.

9. The method of claim 8 wherein the fluidized mass of adsorptiveparticles is held as a plurality of fluidized, vertically contiguousbeds.

10. The method of claim 8 wherein adsorptive particles containing,respectively, adsorbed moisture and carbon dioxide are separatelywithdrawn from said fluidized mass.

11. The method of recovering thecold content of a product ofrectification in the liquefaction and rectification of air to produceoxygen, which comprises passing downwardly in contact with one side ofheat transfer surfaces a stream of air containing moisture and carbondioxide and a moving bed of adsorptive particles effective for thepreferential adsorption of moisture and carbon dioxide, passing upwardlyin contact with the other side of said heat transfer surfaces a streamof rectification product at an initial temperature below about 2'I0 F.,thereby chilling said air to a temperature below the freezing point ofcarbon dioxide while substantially preventing the deposition of saidcarbon dioxide and said moisture on said heat transfer surfaces,separating the thus chilled air from said moving bed, withdrawing fromsaid moving bed adsorptive particles containing adsorbed moisture andcarbon dioxide, treating the withdrawn adsorptive particles to effectelimination of said adsorbed moisture and carbon dioxide, and returningthe treated adsorptive particles to said moving bed.

12. The method of cooling a gaseous stream comprising condensiblematerial to a temperature at which said condensible material freezes,which comprises passing cocurrently said gaseous stream and aparticulate adsorbent adapted to adsorb preferentially said condensiblematerial in contact with one side of heat transfer surfaces and alongthe length of said surfaces, passing in contact with the other side ofsaid surfaces a cooling stream at an initial temperature below thefreezing point of said condensible material, said cooling stream flowingcountercurrently to said gaseous stream, thereby cooling said gaseousstream to said temperature at which said condensible material freezeswhile substantially preventing the deposition of said, condensiblematerial on said surfaces, separating the thus cooled gaseous streamfrom said particulate adsorbent containing adsorbed condensible mate-REFERENCES CITED rial, treating the separated particulate adsorbent Thefollowing references are of record in the to eifect elimination of saidadsorbed condensible me of this patent:

material, and returning the treated particulate adsorbent to passage incontact with said one 5 UNITED STATES PATENTS side of said surfaces.Number Name Date 13. The method of claim 12 wherein the con- 1,825,707Wagner 7- Oct. 6, 1931 densible material comprises carbon dioxide.2,484,875 Cooper Dec. 18, 1949 14. The method of claim 13 wherein thegase- FOREIGN PATENTS Number Country Date PERCIVAL KEITH- 587,774 GreatBritain May 6, 1947 ous stream is air. 10

